[ELF] Emit only one error if -z max-page-size without value

[jhenderson]

Summary

In rL305364, @ruiu changed the mechanism that parses -z option values slightly. This caused a bug, as demonstrated by this test, which now fails:

# REQUIRES: x86
# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
# RUN: ld.lld %t.o -o %t -z max-page-size

.global _start
_start:
  nop

Before, the link succeeded and set the max-page-size to the target default.

After we get the following two error messages:
"invalid max-page-size: "
"max-page-size: value isn't a power of 2" (because an uninitialised variable ends up being passed back to getMaxPageSize).

I think we should have the first error, but not the second, as demonstrated by the new test in this change.

[ruiu]

LGTM

lld/trunk/ELF/Driver.cpp

static bool hasZOption(opt::InputArgList &Args, StringRef Key) {
   return false;
 }
 
 static uint64_t getZOptionValue(opt::InputArgList &Args, StringRef Key,
                                 uint64_t Default) {
   for (auto *Arg : Args.filtered(OPT_z)) {
     std::pair<StringRef, StringRef> KV = StringRef(Arg->getValue()).split('=');
     if (KV.first == Key) {
-      uint64_t Result;
+      uint64_t Result = Default;
       if (!to_integer(KV.second, Result))
         error("invalid " + Key + ": " + KV.second);
       return Result;
     }
   }
   return Default;
 }
 

lld/trunk/test/ELF/invalid-z.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: not ld.lld %t.o -o %t -z max-page-size 2>&1 | FileCheck %s
+# CHECK: invalid max-page-size
+# CHECK-NOT: error
+
+.global _start
+_start:
+  nop